Electric propulsion units

ABSTRACT

A propulsion unit for propelling a waterborne vessel includes an electric motor arranged to provide propulsion, and a housing arranged to contain the motor. The interior of the housing is maintained at an increased pressure of above roughly 2 bar to increase the cooling effect of a cooling gas maintained therein. An end region of the housing is provided with a heat exchange mechanism arranged to cool a cooling gas passing thereover. The heat exchange mechanism is associated with the housing such that, in use, water surrounding the housing absorbs heat therefrom.

FIELD OF THE INVENTION

This invention relates to an improved propulsion unit, and an improvedmethod of cooling such a propulsion unit.

BACKGROUND OF THE INVENTION

It is well known to provide propulsion units that are suspended belowthe hull of a ship in order to provide the ship with propulsion.Examples of such propulsion units are shown in U.S. Pat. Nos. 6,231,407and 5,101,128.

The unit shown in U.S. Pat. No. 6,231,407 contains an electric motorhaving a permanent magnet rotor. Such motors are attractive because theysuffer from low losses in the rotor and therefore do not generateexcessive heat. Such heat generated by the rotor can relatively easilybe dissipated by conduction/convection to the remainder of the unit.However, such motors are disadvantageous because the permanent magnetsused to generate the rotor magnetic field are expensive, it is hard toassemble the rotor, and a suitable power converter system must beemployed. The power converter system must be able to cope with largechanges in power factor with load, which such a motor produces becausethere is no control of the rotor excitation.

U.S. Pat. No. 5,101,128 shows a further example of a propulsion unitusing water cooling, which incorporates a bellows and filter to equalisewater pressure, from outside of the unit. The arrangement provided iscomplex, which leads to increased manufacturing and maintenance costs.Increased maintenance is a particular disadvantage due to the pooraccess to the units, and therefore, it is generally desired to make theunits as simple as possible.

It is an aim of the present invention to overcome, or at least reduce,the problems of the prior art.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided apropulsion unit arranged to propel a water borne vessel comprising anelectric motor, arranged to provide propulsion, and a housing, arrangedto contain the motor, the interior of the housing being maintained at anincreased pressure of above roughly 2 bar to increase the cooling effectof a cooling gas maintained therein.

An advantage of such an arrangement is that it provides a simplestructure that is thus easier to maintain than some prior art propulsionunits.

Preferably, the motor is an induction motor. An advantage of such apropulsion unit is that it provides a mechanically simple and robustmotor.

An agitator may be provided to circulate cooling gas within the housing.Such an agitator is advantageous because it forces circulation of thecooling gas, which can lead to increased cooling.

In the preferred embodiment the agitator comprises at least one fan, butother mechanical equivalents may also be possible. Such a fan may bedriven by a motor, or may be driven by a mechanical link to the motor ofthe unit. The mechanical link may be by any suitable means: drive belt,gears, chain drive, shaft drive, or the like.

The cooling gas may be driven longitudinally along a rotor of the motor.Driving gas along the rotor in this manner may lead to efficient coolingof the rotor.

In one embodiment the rotor comprises a cylindrical body mounted upon ashaft having an axis of rotation, the arrangement being such that afirst cooling passage is provided between the cylindrical body and theshaft. An advantage of such cooling passages is that they provide aconvenient way of conducting cooling gas along the rotor.

The cylindrical body mounted upon the shaft may comprise the rotorwindings, which preferably has a second cooling passage therethrough.Again an advantage of such cooling passages is that they provide aconvenient way of conducting cooling gas along the rotor.

In one, perhaps the preferred, embodiment the unit is arranged such thatcooling gas passes along the first cooling passage in a first direction,and along the second cooling passage in a second direction, differentfrom the first. Preferably, the first direction is roughly in anopposite direction to the second. Such an arrangement is advantageousbecause it provides a clear circulation of gas around the unit, whichmay increase the cooling efficiency.

Preferably, the unit is arranged such that a heat exchange region isprovided between the first cooling passage and the second coolingpassage. Such an arrangement is advantageous because is allows heat tobe removed from the gas passing along either of the cooling passages.

The heat exchange region preferably comprises fins, or the like,provided on an internal surface of a housing of the unit. This providesa simple structure that provides heat exchange; it will be appreciatedthat, in use, the unit will be submersed in water and that therefore,the housing of the unit will be in contact with the water providing agood heat exchange medium. The advantage of the fins is that theefficiency of the heat exchange with the housing is increased.

The fins may be fabricated from a material having a high thermalconductivity, such as aluminium, or copper. Alternatively, oradditionally the fins may be fabricated integrally with the housing.Fabricating the fins integrally is advantageous due to the ease ofmanufacture that such an arrangement would bring.

The fins may be provided in an end region of the housing. Such anarrangement is advantageous because the end region provides a convenientarea having a relatively large contact with water outside the unit, andtherefore, provides a good capacity for removing heat from the fins.

Conveniently, the unit is arranged such that gas flowing in the seconddirection, exits the second cooling passage into the heat exchangeregion. An advantage of this flow direction is that gas exiting thesecond passage is likely to be hotter than that exiting the firstcooling passage, and it is therefore advantageous to provide heatexchange for this gas.

The stator may include end turns, which protrude from the stator, andconveniently the unit is arranged such that air exiting/entering eitherof the cooling passages passes over the end turns. This is advantageousbecause it provides extra cooling for the end turns.

The pressure may be in the range of roughly 2 bar to roughly 7 bar. Thisrange is believed suitable because, pressures below 2 bar are believedto provide negligible increased benefit, whilst pressures above 7 bardisproportionately increase the cost of the propulsion unit and requireexcess power to circulate the gas therein.

Such an arrangement is advantageous because the increased pressure helpsto remove the pressure gradient across seals between the outside and theinside of the unit (for example around the shaft on which the propelleris mounted). Reduction of the pressure gradient helps to make the sealand prevent water from entering the unit. Further, the increasedpressure can be used to help drain water that has entered the unit bydriving the water back up a stay connecting the unit to the vessel towhich it is attached. The skilled person will appreciate that theincreased pressure improves the heat transfer, since the heat transfercoefficient (h) increases significantly with air density (typicallyh∝ρ^(0.64)).

Conveniently, the stator is mounted such that it is in good thermalcontact with the inside surface of a housing for the unit. Such anarrangement provides efficient cooling of the stator. Good thermalcontact would generally mean in contact, but is intended to coversituations in which a high thermal conductivity member is placed betweenthe stator and the housing.

Preferably, the cooling gas is air, but any suitable cooling gas may beused. For example CO₂, nitrogen, refrigerant gases, noble gases, etc.may be used. Air is advantageous because it is cheap, readily available,and non-toxic.

According to a second aspect of the invention there is provided apropulsion unit arranged to propel a water borne vessel comprising anelectric motor, arranged to provide propulsion, said unit furthercomprising a housing that contains the motor, wherein an end region ofthe housing is provided with a heat exchange mechanism arranged to coolcooling gas passing thereover, the heat exchange mechanism beingassociated with the housing such that, in use, water surrounding thehousing absorbs heat therefrom.

Such a unit is advantageous because it is mechanically simple and yetcan provide effective cooling for the motor. It will be appreciatedthat, in use, the housing is surrounded by water and is therefore,itself, readily cooled, and can absorb heat from the heat exchangemechanism.

The heat exchange mechanism may comprise one or more fins provided on aninterior surface of the housing, and therefore provides a large surfacearea over which heat exchange can occur.

The fins may be fabricated from a material having a high thermalconductivity such as aluminium or copper, thereby increasing the rate ofheat exchange between the cooling gas and the housing.

The fins may be fabricated integrally with the housing, and as suchprovide a structure that is simple to fabricate.

In some embodiments the motor comprises a stator that includes end turnsthat protrude from the stator. In such cases the unit may be arrangedsuch that the cooling gas passes over the end turns before passing theheat exchange mechanism. Such an arrangement can provide efficientcooling of stator end windings, which is advantageous, because these endwindings will not be in closely associated with the housing and will notbe as well cooled as the remainder of the stator.

The motor comprises a rotor, which may comprise a cylindrical bodymounted upon a shaft having an axis of rotation, the arrangement beingsuch that a first cooling passage is provided between the cylindricalbody and the shaft.

The cylindrical body mounted upon the shaft may comprise the rotorwindings. The rotor windings may have a second cooling passagetherethrough.

Preferably the unit is arranged such that cooling gas passes along thefirst cooling passage in a first direction, and along the second coolingpassage in a second direction, different from the first.

It is convenient if the first direction is roughly in an oppositedirection to the second. Such an arrangement provides a convenientcircuit for circulating air within the housing, which provides efficientcooling of the rotor.

Preferably, the pressurised gas is at any pressure between roughly 2 barand roughly 7 bar i.e. it may be roughly any of the following pressures:2, 3, 4, 5, 6, 7 bar.

The motor may be an induction motor, or may be a synchronous motor.

Conveniently, the pressurised gas is air, but any suitable gas may beused. For example CO₂, nitrogen, refrigerant gases, noble gases, etc.may be used. Air is advantageous because it is cheap, readily available,and non-toxic.

The pressurised gas may be circulated by at least one agitator, whichconveniently comprises a fan. An advantage of such an agitator is thatit forces movement of the gas, which in turn moves heat past heatexchange surfaces, increasing the cooling effect.

According to a third aspect of the invention there is provided apropulsion unit arranged to propel a water borne vessel comprising anelectric motor arranged to provide propulsion, said unit furthercomprising a housing that contains the motor, wherein an end region ofthe housing is provided with a heat exchange mechanism arranged to coolcooling gas passing thereover and wherein the interior of the housing ismaintained at an increased pressure of about roughly 2 bar to increasethe cooling effect of a cooling gas maintained therein.

According to a fourth aspect of the invention there is provided a methodof cooling a propulsion unit comprising a motor provided in a housingand arranged to drive a propeller for propulsion, comprisingpressurising a gas within the housing in order to increase the coolingprovided by the gas.

According to further aspects of the invention there are provided amethod of cooling a propulsion unit according to any of the earlieraspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

There now follows by way of example only a detailed description of theinvention with reference to the accompanying drawings of which:

FIG. 1 is a side elevation of one embodiment of a propulsion unitaccording to the present invention;

FIG. 2 is a cross section along line AA of FIG. 1;

FIG. 3 is an enlargement of a portion of FIG. 2;

FIG. 4 is an enlargement of a portion of FIG. 1 showing a bafflearrangement;

FIG. 5 shows a side elevation of a second embodiment of a propulsionunit according to the present invention;

FIG. 6 is an end elevation of the housing for the embodiments shown inFIGS. 1 and 5;

FIG. 7 is a perspective view of a portion of the housing shown in FIG.6;

FIG. 8 shows a cross section through the rotor of a motor of FIG. 1; and

FIG. 9 shows an alternative arrangement for the embodiment of thepropulsion unit shown in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The propulsion units 2 shown in FIGS. 1 and 2 have a housing 4 arrangedto be suspended from beneath a hull of a ship (not shown) by a stay 6.The stay 6 includes at a top region thereof slip rings 8, which allowthe housing 4 and stay 6 to be rotated through 360° to providedirectional thrust for the ship to which the arrangement is attached. Apropeller 10 is driven by a motor 12 contained within the housing 4.

The motor 12 comprises an electrical induction motor having a stator 14,and a rotor 16. The rotor 16 comprises a central shaft 18 supported bybearings 20,22. The central shaft 18 supports, using supporting arms 26,a cylindrical body comprising rotor windings 24, and a cross section ofa rotor is shown in FIG. 8. The spaces between the arms 26 provide afirst cooling passage, which will be described hereinafter. A secondcooling passage is provided by ducts 27 within the rotor windings 24 andprovides a cooling path longitudinally along the rotor. The stator 14comprises a winding 28, the stator being a shrink or press fit into thehousing 4. Alternatively, a clearance fit may be used in conjunctionwith a high thermal conductivity adhesive to fix the stator within thehousing. End turns 30,32,34,36 of the field winding 28 are shown at endregions thereof. Power to the winding 28 is supplied via cables (notshown) which pass along the stay 6 from the ship. An air gap 38 (ofroughly 4 mm in this embodiment) exists between the stator 14 and therotor 16. It will be appreciated that such an air gap is not largeenough to allow convection.

The winding 28 is of conventional design. However, the winding 28 ismounted in close proximity to the housing 4, and thus water surroundingthe housing provides sufficient cooling for the winding 28.

Turning specifically to the propulsion unit shown in FIG. 1, an endregion 40 of the propulsion unit 4 comprises roughly hemi-spherical endcap that is mounted upon a cylindrical wall. A plurality of heatexchange fins 42 are arranged in a roughly equispaced manner around theinside wall of an end region of the cylindrical wall, adjacent thehemi-spherical shell, and the arrangement of these fins 42 is moreclearly shown in FIG. 3. A baffle 44 comprising a band, a first edge ofwhich runs around a first radius from the rotor, and a second edge ofwhich runs around a second radius from the rotor, is positioned suchthat its edge having the greater radius is positioned adjacent the heatexchange fins 42. The baffle 44 is provided in a heat exchange region45, helps to lengthen the path of cooling air, as will be explainedhereinafter, and directs air through the heat exchange fins 42. A baffle46 of a similar shape to the previously described baffle 44 is providedon an exit of the heat exchange fins 42, again to direct the flow ofcooling air.

FIG. 9 shows an alternative arrangement for the propulsion unit shown inFIG. 1, in which the baffle 44 of FIG. 1 has been extended in length andcurved to form the baffle 58, in the heat exchange region 59 shown inFIG. 9. This extension of the baffle 58 lengthens the air path and keepsthe cooling air in contact with the hemispherical portion 40 of thehousing 4 for longer. Further, the heat exchange fins 42 of FIG. 1 havelengthened to form fins 60 of FIG. 9. This lengthening provides anincreased heat exchange area for the length of the path defined by thebaffle 58.

A fan motor 48 provided in the stay 6 is arranged to drive a centrifugalfan 50 to provide an agitator to circulate air through the motor 12. Afurther partition 52 runs along a central region of the stay 6 in orderto divide the stay 6 into an inlet portion 54 for cooling air, and anoutlet portion 56 for cooling air. (It will be appreciated that thisrefers to inlet and outlet of the stay 6 since the propulsion unit 2 issubmerged underneath a ship, and that air is circulated around theinside of the housing 4 and stay 6).

In use, the fan motor 48 drives a centrifugal fan 50 to draw air intothe air inlet portion 54, through the fan 50 and out of the stay 6through the outlet portion 56. Once the air has passed from the outletportion 56 into the housing 4 it passes along the ducts 27 within therotor windings 24. The partition 52 extends into the housing 4 and isbrought into close proximity with the rotor 16 to ensure that airpassing from the outlet portion 56 is so directed. As air passes intothe rotor windings 24 it passes the end turns 32,34 of the winding 28and thus provides a cooling action thereto. Further, as air exits therotor windings 24 is passes the end turns 30,36 of the stator winding.Further, air exiting the rotor winding 24 is directed by the baffle 46through the heat exchange fins 42 (60 in FIG. 9), where the air iscooled via heat exchange with these fins 42,60, which are in goodthermal contact with the housing 4. The baffle 44 (58 in FIG. 9) ensuresthat air flow is maintained in contact with the heat exchange fins 42,60for a predetermined period, before passing back along the rotor 16through the spaces between the supporting arms 26. After exiting therotor 16 the air is drawn up in to the inlet portion 54 of the stay 6.

Turning to the embodiment shown in FIG. 5, it will be noted that themain difference is that air is forced to circulate longitudinally of therotor 16. Instead a plurality of fans, or agitators, are used to locallyforce convection as will now be described.

A fan 62,64,66,68 is positioned adjacent each of the end turns30,32,34,36 of the stator winding 28 and forces local circulation of theair therethrough. A partition 70 extends up into the stay 6 such thatair is forced to circulate by the fan 66 in proximity to the wall of thestay 6, which improves the cooling provided by the fan 66.

Air within the housing 4 and stay 6 is maintained at a pressure aboveatmospheric. Pressures in the range of 2 bar to 7 bar are believed to beadvantageous. This increased pressure can also usefully be used to helpremove seawater from a bottom region (generally referred to as a sump)of the housing, and also helps to maintain a seal around the portion ofthe rotor 16 passing through the housing 4 to the propeller 10.

1. A propulsion unit for propelling a waterborne vessel, comprising: a)an electric motor for providing propulsion, the motor including a rotorhaving a cylindrical body mounted upon a shaft having an axis ofrotation, the cylindrical body including rotor windings, a first coolingpassage being provided between the cylindrical body and the shaft, and asecond cooling passage being provided through the rotor windings; b) ahousing for containing the motor and bounding an interior through whicha cooling gas is passed longitudinally of the rotor along the firstcooling passage in a first direction and along the second coolingpassage in a second direction different from, and opposite to, the firstdirection, the interior being maintained at an increased pressure ofabove roughly 2 bar to increase a cooling effect of the cooling gas; andc) a heat exchange region including fins provided on an internal surfaceof the housing between the first cooling passage and the second coolingpassage.
 2. The unit according to claim 1, wherein the fins areconstituted of a material having a high thermal conductivity.
 3. Theunit according to claim 1, wherein the fins are associated with thehousing such that, in use, water surrounding the housing absorbs heatfrom the fins.
 4. The unit according to claim 1, wherein the fins areintegral with the housing.
 5. The unit according to claim 1, wherein thefins are provided in an end region of the housing.
 6. The unit accordingto claim 1, wherein the cooling gas flowing in the second directionexits the second cooling passage into the heat exchange region.
 7. Theunit according to claim 1, wherein the motor also comprises a statorthat includes end turns that protrude from the stator, and wherein theunit is arranged such that the cooling gas passing along the coolingpassages passes over the end turns.
 8. The unit according to claim 7,wherein the stator is mounted in good thermal contact with an insidesurface of the housing.
 9. The unit according to claim 1, wherein thecooling gas is air.
 10. The unit according to claim 1, wherein thepressure within the housing is between roughly 2 bar and roughly 7 bar.11. The unit according to claim 1, wherein the motor is one of aninduction motor and a synchronous motor.
 12. The unit according to claim1, and an agitator for circulating the cooling gas within the housing.13. The unit according to claim 12, wherein the agitator comprises atleast one fan.
 14. A propulsion unit for propelling a waterborne vessel,comprising: a) an electric motor for providing propulsion, the motorincluding a stator that includes end turns that protrude from thestator, and a rotor having a shaft that has an axis of rotation; and b)a housing for containing the motor, the housing having an axial endregion provided with a heat exchange mechanism arranged to cool acooling gas passing thereover, the heat exchange mechanism comprising atleast one fin provided on an interior surface of the housing such that,in use, water surrounding the housing absorbs heat therefrom, thecooling gas passing over the end turns before passing the heat exchangemechanism.
 15. The unit according to claim 14, wherein said at least onefin is constituted of a material having a high thermal conductivity. 16.The unit according to claim 14, wherein said at least one fin isintegral with the housing.
 17. The unit according to claim 14, whereinthe rotor comprises a cylindrical body mounted upon the shaft, andwherein a first cooling passage is provided between the cylindrical bodyand the shaft.
 18. The unit according to claim 17, wherein thecylindrical body mounted upon the shaft comprises rotor windings for therotor.
 19. The unit according to claim 18, wherein the rotor windingshave a second cooling passage therethrough.
 20. The unit according toclaim 19, wherein the cooling gas passes along the first cooling passagein a first direction, and along the second cooling passage in a seconddirection, different from the first direction.
 21. The unit according toclaim 20, wherein the first direction is roughly in an oppositedirection to the second direction.
 22. The unit according to claim 14,wherein the housing has an interior maintained above a pressure ofroughly 2 bar.
 23. A propulsion unit for propelling a waterborne vessel,comprising: a) an electric motor for providing propulsion withconcomitant generation of heat, the motor including a stator and a rotorrotatable about an axis; b) a housing for containing the motor, thehousing having a front end region which, in use, is submerged in ambientwater and is in a heat exchange relationship with the ambient water; andc) means for circulating a cooling gas within the housing, includingmeans for directing the cooling gas past the motor to absorb the heatgenerated therefrom, and for directing the cooling gas after heatabsorption into the front end region for transferring the heat to theambient water, the circulating means including fins for directing thecooling gas along the axis, and baffles for deflecting the cooling gastransversely of the axis and into the front end region.
 24. The unitaccording to claim 23, and a plurality of cooling passages extendingthrough the motor along the axis, and wherein the fins and the bafflesare located at at least one end of the cooling passages.